I know it is probably going to sound outlandish but... where does space finally stop

Where does space finally stop being space. Could there be an end to it if it is a vacuum? If there isn't an end then could there be different spots with universes? Different types of being entirely? Maybe non atomic or no hands or feet like? Could there be people human or non human out there that would resemble humanity? Does anyone live being the hard finish we have of 100 years and past it? Could they be technological or horse sword and outhouse? Could they know of wealth we don't or sciences or other things we just can not conceive of? Do you think in our evolution and basic guessing about how to live that we would impress them? Should we care?

Given that c is the maximum speed limit of the universe, the edge of our universe is roughly 13Gigayears (however old the universe actually is) away from us in any direction. Anything past that has no meaningful definition to us, as we could never, even given infinite time, get past that.

this confuses me and I will read the wiki article in a bit but if space was space as in nothing there then how did the big bang which produced matter produce space?

Space and time are the same thing in our four dimensional Universe. What we have is an expanding bubble of dimensions. You can't exactly ask "what's 10 km outside this bubble", because you can't measure km without space or time. The metre is defined by how much space is crossed in a unit of time. Without them, you don't have any distance. So even thinking about what can happen (can't happen without time) outside this bubble (can't be an outside without distance) is just silly.

As the Universe cannot expand infinitely quickly (or everything embedded in spacetime would also be spread out to zero density), it follows that the Universe is finite in volume. If it is finite in space, it must also be finite in time.

Space isn't nothing, it's our distances and our times. We take them for granted as we can't grab or eat them, but they're even more real than a big solid rock, because that rock is 99.98% nothing and spacetime is 100% everything.

Given that c is the maximum speed limit of the universe, the edge of our universe is roughly 13Gigayears (however old the universe actually is) away from us in any direction.

No, the universe is much larger, as that wikipedia page explains:

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The region visible from Earth (the observable Universe) is a sphere with a radius of about 47 billion light years...

The current estimate of the Universe's age is 13.72±0.12 billion years old, based on observations of the cosmic microwave background radiation.[39] Independent estimates (based on measurements such as radioactive dating) agree at 13–15 billion years.[40] The Universe has not been the same at all times in its history; for example, the relative populations of quasars and galaxies have changed and space itself appears to have expanded. This expansion accounts for how Earth-bound scientists can observe the light from a galaxy 30 billion light years away, even if that light has traveled for only 13 billion years; the very space between them has expanded. This expansion is consistent with the observation that the light from distant galaxies has been redshifted; the photons emitted have been stretched to longer wavelengths and lower frequency during their journey. The rate of this spatial expansion is accelerating, based on studies of Type Ia supernovae and corroborated by other data.

Since the universe increases it's diameter more rapidly then light can pass through it, the universe itself is much larger then just the age and speed of light would suggest.

I don't know whether we'll ever reach the same degree of confidence that we have in, say evolution, but yes, scientists have some interesting ideas about the existence of 9 or 10 dimensions, of which time is one. There's also a fascination notion that there are an infinite number of universes, in which every possible configuration of mass exists.

I'm halfway through Brian Greene's The Hidden Reality, his latest recap of the current state of string theory. You might want to give it a try; it will answer your questions and raise a lot more you haven't thought of.

I don't like these answers. For all we know, the universe is infinite. It could just keep on going. The observable universe, that we can see, is finite. But there's no evidence that it doesn't go on forever.

I don't like these answers. For all we know, the universe is infinite. It could just keep on going. The observable universe, that we can see, is finite. But there's no evidence that it doesn't go on forever.

It would have needed to exist forever in order to extend forever.

This is quite obviously false. You propose infinite spacetime, you can't have the space bit without the time bit.

I don't like these answers. For all we know, the universe is infinite. It could just keep on going. The observable universe, that we can see, is finite. But there's no evidence that it doesn't go on forever.

It would have needed to exist forever in order to extend forever.

This is quite obviously false. You propose infinite spacetime, you can't have the space bit without the time bit.

Hat has it right here. In simplest terms: the universe has existed for finite time, and expands at a finite rate. Its size is therefor finite.

As for the observable universe, it continues to grow in spatial terms, but it actually continues to shrink in matter/energy terms, as objects pass our horizon never to be seen again.

I don't like these answers. For all we know, the universe is infinite. It could just keep on going. The observable universe, that we can see, is finite. But there's no evidence that it doesn't go on forever.

It would have needed to exist forever in order to extend forever.

This is quite obviously false. You propose infinite spacetime, you can't have the space bit without the time bit.

Hat has it right here. In simplest terms: the universe has existed for finite time, and expands at a finite rate. Its size is therefor finite.

As for the observable universe, it continues to grow in spatial terms, but it actually continues to shrink in matter/energy terms, as objects pass our horizon never to be seen again.

You guys are dismissing this in a non-convincing way.

The observable universe is finite. But civilizations at the edge of the observable universe have their own observable regions that extend beyond our observable universe. How far does that go if it's not infinite? I'm not aware of any way to know.

I don't think anyone's saying the universe expanded from finite to infinite, but how do we know the big bang wasn't infinite? Or if it is finite, how far does the universe go before it wraps back on itself or whatever?

The observable universe is finite. But civilizations at the edge of the observable universe have their own observable regions that extend beyond our observable universe. How far does that go if it's not infinite? I'm not aware of any way to know.

There's no way to just look and see, ala a naive direct measurement, of course. However thanks to the magic of deductive reasoning, we don't always have to resort to such, which is why you're getting the above explanations.

If the popular version of the creation of the universe is to be accepted (the big bang), the universe came into existence as a finite point containing a finite amount of matter and energy. Enormous, but finite. It then set about rapidly expanding. From this point of view, it is clear that the universe is finite.

However, some people when talking about the size of the universe are interested in geometry. As in, they want to know whether you can keep traveling in one direction forever. The current thinking is that the universe is flat*. If this is the case, the universe will continue to expand infinitely - that doesn't mean the universe is infinite though. It means there is no limit on its expansion. It is of finite (but huge) size, but it will keep getting bigger and bigger without limit.

If you hear a cosmologist say that the universe is infinite, she is probably just being a little sloppy in describing a non-positive curvature. She doesn't mean that there is "stuff" extending out infinitely in all directions, she means that stuff is allowed to expand out infinitely in all directions. A very important distinction - the former being a statement about the current state, the latter being a statement about future behavior and the nature of the "end" of the universe.

*(according to WMAP data, within 2% of flat, but of course even slightly positive curvature fundamentally changes the end-game for the universe)

The observable universe is finite. But civilizations at the edge of the observable universe have their own observable regions that extend beyond our observable universe. How far does that go if it's not infinite? I'm not aware of any way to know.

Me neither, but seeing how our current models hold the Universe's expansion to be isotropic, it would be reasonable to expect observers at the edge of our own observable region to see a night sky of similar size and matter distribution.

Apply induction. Asplode brain.

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I don't think anyone's saying the universe expanded from finite to infinite, but how do we know the big bang wasn't infinite?

I would think the finite energy of the Cosmic Microwave Background is proof the Big Bang wasn't infinite.

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Or if it is finite, how far does the universe go before it wraps back on itself or whatever?

Those people studying the very early universe's inflationary period and its associated (hypothetical) inflaton really need to get cracking.

The observable universe is finite. But civilizations at the edge of the observable universe have their own observable regions that extend beyond our observable universe. How far does that go if it's not infinite? I'm not aware of any way to know.

Our observable universe isn't the same as someone else's, this is true, but that someone else also has to exist within spacetime and the cool bit is no matter which way they look, they will see a younger universe until they're looking so far they see the surface of last scattering - the CMB. We've been at that level for 20 years or so now.

To any observer, the observable universe is a 45-50 billion light year bubble centered on himself. There's no "edge" to it, as the observer sees the universe's history around him.

Which suggests the question, how can a finite universe that isn't closed have everyone see a similar sky in every direction no matter their location without being infinite?

I'm not saying you could travel that distance. Expansion is accelerating, we're talking about points that are moving away from us faster than light speed. But the people living in distant galaxies have a similar sky, and the people living in galaxies distant to them do too, so we have no idea how far the universe extends.

Apteris wrote:

Me neither, but seeing how our current models hold the Universe's expansion to be isotropic, it would be reasonable to expect observers at the edge of our own observable region to see a night sky of similar size and matter distribution.

Exactly.

Apteris wrote:

I would think the finite energy of the Cosmic Microwave Background is proof the Big Bang wasn't infinite.

The CMB dates from when the universe became transparent to light. It would look like that either way.

Our observable universe isn't the same as someone else's, this is true, but that someone else also has to exist within spacetime and the cool bit is no matter which way they look, they will see a younger universe until they're looking so far they see the surface of last scattering - the CMB. We've been at that level for 20 years or so now.

To any observer, the observable universe is a 45-50 billion light year bubble centered on himself. There's no "edge" to it, as the observer sees the universe's history around him.

Yes yes yes, we get that. This is exactly the answer I was trying to preempt because it doesn't answer the question.

If they have their bubble, and we have ours, there's clearly more universe than we can observe. These regions might be too distant to observe each other, but nobody is going to claim the matter that generated the CMB we see did anything other than evolve into galaxies and civilizations like ours, or that the civilizations there see a non-similar sky.

So even if they're not mutually observable, the universe either has infinite size (though not infinite density) or has a topology that preserves the "no edges" property without being infinite. Either way there's a lot more universe than is observable to us.

This not being my area of expertise, I hesitate to say too much about the geometry of spacetime at such a shock front, however since we lose isotropy and homogeneity we necessarily lose constant curvature, so it isn't a simple shape.

Separately, as I recall from differential geometry, it is also possible for a flat universe to be compact - for example it could be a 4-torus. So you could have finite size without a shock front, too.

So even if they're not mutually observable, the universe either has infinite size (though not infinite density) or has a topology that preserves the "no edges" property without being infinite. Either way there's a lot more universe than is observable to us.

Since we were basically at the center of the universe before most expansion happened, isn't most of the volume of the universe still visible to us (at some point in its history at least)?

Separately, as I recall from differential geometry, it is also possible for a flat universe to be compact - for example it could be a 4-torus. So you could have finite size without a shock front, too.

This is exactly what I'm getting at. And whatever its dimensions, this shape must be much lager than the dimensions of the observable universe. I know we don't have evidence for this other than lower bounds we could place on the shape's dimensions from the lack of shape we can see in the observable universe, I just wanted to get to the meat of this question as opposed to being stuck on the generic Discovery channel "size of the observable universe" answer.

redleader wrote:

Since we were basically at the center of the universe before most expansion happened, isn't most of the volume of the universe still visible to us (at some point in its history at least)?

As far as we can tell there is no absolute center, and the extent of the universe is probably much larger than the observable universe. It could go for trillions of light years in every direction, but the universe isn't old enough, and is expanding too fast, for us to ever be able to see that.

I don't like these answers. For all we know, the universe is infinite. It could just keep on going. The observable universe, that we can see, is finite. But there's no evidence that it doesn't go on forever.

It would have needed to exist forever in order to extend forever.

This is quite obviously false. You propose infinite spacetime, you can't have the space bit without the time bit.

I don't think that's how infinity works.

The universe could start out with infinite size and very high density. It could then expand for a finite time until it reached the density we have now and it would look just like a big bang.

The question of whether the universe is infinite or not (not just unbounded) doesn't appear to have been solved just yet.

This is exactly what I'm getting at. And whatever its dimensions, this shape must be much lager than the dimensions of the observable universe. I know we don't have evidence for this other than lower bounds we could place on the shape's dimensions from the lack of shape we can see in the observable universe, I just wanted to get to the meat of this question as opposed to being stuck on the generic Discovery channel "size of the observable universe" answer.

Well, if the universe is a flat closed manifold then the only lower bound I can think of is that we don't seem to be able to observe the same object looking in two different directions - or at least, not enough objects for there to be identifiable structure. If it is a 4-torus, that sets a lower bound on the minor circumference at ~100 Gly.

If it turns out to be a 4-sphere instead (positive curvature) it must be one hell of a lot bigger, because as yet we can't distinguish the curvature from flat.

Frennzy wrote:Given that c is the maximum speed limit of the universe, the edge of our universe is roughly 13Gigayears (however old the universe actually is) away from us in any direction.

That would be the size of the visible universe. Not the entire universe

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Which suggests the question, how can a finite universe that isn't closed have everyone see a similar sky in every direction no matter their location without being infinite?

Because the universe is not a three dimensional construct. You are apply three dimensional thinking to a construct with more than 3 dimensions. It doesn't work.

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Our universe is what resulted from the Big Bang. Any "void" that this is expanding in to is outside our universe.

This is wrong. The universe is not expanding into anything. You can't even say the universe is expanding, precisely because there is nothing for it to expand into. What is happening is that everything is rushing apart.

Once again, you are apply three dimensional thinking to a non three dimensional space.

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If a person was transported beyond the "front" of this expansion, what would this person see? Could this person even exist "there"?

There is no front. There is no edge.

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redleader wrote:Since we were basically at the center of the universe before most expansion happened, isn't most of the volume of the universe still visible to us (at some point in its history at least)?

there is no center of the universe. Or, equally, every single point in the universe can equally be called the center.

This stuff quickly stops making sense when the universe is conceptualized as a three dimensional space. It's not. It's fun to talk about at college parties, but you pretty soon run into people just spouting nonsense.

That would be the size of the visible universe. Not the entire universe

It wouldn't be either. The observable universe has a radius of about ~45 Gly, thanks to expansion.

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Which suggests the question, how can a finite universe that isn't closed have everyone see a similar sky in every direction no matter their location without being infinite?

Because the universe is not a three dimensional construct. You are apply three dimensional thinking to a construct with more than 3 dimensions. It doesn't work.

He obviously knows that, or he wouldn't be talking about closure. His question is exactly on point, and the answer to it is: it can't.

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This is wrong. The universe is not expanding into anything. You can't even say the universe is expanding, precisely because there is nothing for it to expand into. What is happening is that everything is rushing apart.

That's a pretty pedantic point, and not even really a correct one. Expanding doesn't require anything to "expand into." Expanding is absolutely the commonly accepted terminology. As for whether it is expanding into anything, there's no way to know. Something that is outside of our universe is by definition not something we can investigate. Is there a four dimensional "space" into which our universe is embedded? Or is our model of the universe as the three dimensional surface of a four dimensional shape just that - a mathematical model?

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There is no front. There is no edge.

I'm not sure this is a resolved question, although you're certainly right that the currently accepted model doesn't have one. Thanks to this discussion I went digging through old journals a bit yesterday because I remembered reading a paper about a (completely mathematically consistent) model that treated the big bang as a more typical explosion - with a shock front. I didn't find it though. =/

That's a pretty pedantic point, and not even really a correct one. Expanding doesn't require anything to "expand into." Expanding is absolutely the commonly accepted terminology.

I don't think it's pedantic, it's a pretty salient point.

In order to say spacetime is expanding, you need to have a separate fabric of reality for spacetime to reside in that it can expand into. There isn't (as far as our cosmological models tell us). If there was, that would open up whole new fields of research.

Spacetime is all there is. It cannot get bigger to inhabit a bigger space, there's nothing bigger for it to expand into. It can simply create space as everything in it moves apart. It's pretty unintuitive, but it is what it is.

So our universe is getting larger (or expanding) [or creating more space within itself thereby giving the illusion of expansion] {wait, is it really that different for laymen understanding?} and there is, from what we can determine thus far, absolutely nothing outside our universe. Or I still don't understand... I get that we couldn't measure something "outside" the universe but I guess I assumed there were strange theories that may predict what is out there. Evidently, according to this thread, there aren't?